This invention relates to the field of LED drive circuits, and more particularly to a method for driving an LED array using a sample and hold method to achieve square wave PWM current waveform required by LED loads.
Conventional LED array driver circuits typically use an inexpensive linear voltage regulator to provide a Vcc biasing rail as an energy source for LEDs to be displayed. When an LED is selected for lighting, a semiconductor switching device, such as a transistor, is activated to provide a current path through the selected LED(s). At the end of the display period, the device is then turned off. A significant drawback of such a voltage driving arrangement is that voltage overshoots occur at the time of turn off of the switching device due to continued conduction of the source regulator at the instant of turn off. This event presents a condition wherein a large filtering capacitor is receiving charge from the linear regulator at the instant of removal of the LED loading. Then, at the next turn-on cycle of the LED, a higher voltage is present on the voltage biasing rail, causing large leading current spikes at that voltage transition. These overshoots can be injected into neighboring circuitry with degrading or destructive effect both to the circuits and the LED.
To eliminate such overshoot problems, circuits are configured to create a constant current source from a linear voltage regulator. FIG. 1 shows such a conventional LED driver circuit 10, having a first amplifier 12 for amplifying an ILED signal present at node 14 and a second amplifier 16 for controlling the operation of the LED. In response to a logical switching signal 18, second amplifier 16 presents a regulated voltage signal 20 to transistor 22 to begin current conduction through LEDs 24. The resulting LED current, ILED, is sensed via the sensing resistor 26 and is amplified by first amplifier 12 to provide a current feedback signal to second amplifier 16. The Laplace transfer function of the second amplifier is governed by the equation                                           R            A                                R            B                          ⁢                  1                      1            +                          s              ⁢                              xe2x80x83                            ⁢                              R                A                            ⁢                              C                1                                                                        [        1        ]            
A linear time delay that is provided by the RAC1 impedance combination implements a low pass filter that reduces overshoot in ILED, thereby allowing current signal ILED to follow reference signal 18. A disadvantage of such linear low pass filtering is the large overshoot in LED current during the turn-on of LEDs 24. For applications requiring LEDs 24 to be displayed using a pulse width modulated (PWM) mode at a high frequency, such as 400 Hz, and an exemplary duty cycle of 40%, substantial ripple current results in ILED that can cause degradation in the optical and electrical performance of the LEDs 24. This degradation can include loss of intensity control and accuracy, in addition to creation of noise signals that can interfere with the sampling scheme.
Thus, a need exists for an apparatus that can inhibit the overshoots while allowing high speed selectivity of the devices in an LED array.
According to a preferred embodiment of the present invention, a circuit is provided for synchronizing the current feedback and control signals of an LED driving circuit with a second signal for driving a biasing power converter in order to eliminate voltage and current overshoots associated with LED loading discontinuities. By such synchronization, a switching device in the power converter which is connected to an energy source is inhibited during times when the LED load driving device is inhibited to prevent undesired charging of intermediate buffering capacitances. Through the use of gate clamping devices, a single logic signal, with inversion where appropriate, can cause all switching devices to turn-on and turn-off simultaneously. This invention is applicable to both flyback converters and push-pull converters.
Further, a sample and hold circuit preferably provides for an operating biasing level that will insure that a subsequent turn-on will having the same operating conditions that were present at a previous turn-on. The response time of such a sample and hold circuit will preferably be longer than the time period of the operating frequency of the converter.
The present invention also provides a method for synchronizing the switching LED driver to the switching device in the voltage converter such that both the activation time periods and the inactivation time periods coincide, and that no energy transfer is enabled if an energy load is not present.